swift uiview does not have a member named center

This is my `RectangleView`: ``` import UIKit class RectangleView: UIView { init(frame: CGRect) { // Initialization code super.init(frame: frame) } /* override func drawRect(rect: CGRect) { // Drawing code } */ } ``` I add this `RectangleView` on my `CustomViewController`: ``` import UIKit class CustomViewController: UIViewController { var ballX:Int = 0 var ballY:Int = 0 let BALLWIDTH:Int = 50 let BALLHEIGHT:Int = 50 var navigationHeight:Double = 0.0 var pos:CGPoint? var rectangleView:RectangleView? override func viewDidLoad() { super.viewDidLoad() // Do any additional setup after loading the view, typically from a nib. self.ballX = Int(arc4random_uniform(320)); self.ballY = Int(arc4random_uniform(480)); self.navigationHeight = Double(self.navigationController.navigationBar.frame.size.height) var rect = CGRect(x: 0, y: 0, width: self.BALLWIDTH, height: self.BALLHEIGHT) self.pos = CGPoint(x: self.ballX, y: self.ballY); println("x: \(self.ballX), y: \(self.ballY)") self.rectangleView = RectangleView(frame: rect) self.rectangleView!.backgroundColor = UIColor.redColor() self.view.addSubview(self.rectangleView!) NSTimer.scheduledTimerWithTimeInterval(0.05, target: self, selector: Selector("moveBall"), userInfo: nil, repeats: true) } func moveBall() { self.rectangleView!.center = CGPointMake(self.rectangleView!.center + self.pos!.x, self.rectangleView!.center + self.pos!.y) } ... ``` And I get a error message ``` 'RectangleView?' does not have a member named 'center' ``` Should I define this `center` on my `RectangleView`? My `RectangleView` inherits from `UIView`, so I feel very confused. Thanks your help. Edit I use this code ``` func moveBall() { if let rv = self.rectangleView { rv.center = CGPointMake(rv.center + self.pos!.x, rv.center + self.pos!.y) } ``` I get the new error message ``` 'CGPoint' is not convertible to 'UInt8' ``` I think the `rv.center` type is `UInt8`, but `self.post!.x` is `nsnumber`, so I how could I convert to `CGPointMake` accept type? I have convert `UInt8(self.pos!.x)` and convert calculate result to `CGPointMake` accept type again? Have any other best method? Thanks your help.